In related art, a fixing device that is used for an image forming apparatus of an electrophotographic type has been known which has a cylindrical member, a ceramic heater, a heater frame and a backup roller (for example, refer to U.S. Pat. No. 6,266,510). The ceramic heater is disposed to slidingly contact an inner peripheral surface of the cylindrical member and is configured to heat the cylindrical member. The backup roller has a straight shape and pinches the cylindrical member between the ceramic heater and the backup roller. Also, the heater frame is disposed at one side of the backup roller opposite to another side of the nip plate where the backup roller is provided and supports the heater frame.
In the fixing device, a part of the heater frame facing the backup roller has a curved concave shape so that a central portion thereof in an axial direction of the cylindrical member is more spaced from the backup roller than both end portions thereof in the axial direction. Thereby, upon the fixing, when both end portions of the heater frame in the axial direction are pressed towards the backup roller, a pressure that is applied between the ceramic heater and the backup roller becomes higher at both end portions in the axial direction than at the central portion. Therefore, a conveying speed of a recording sheet, which is conveyed through between the cylindrical member and the backup roller, becomes faster at both end portions of the recording sheet, so that a central portion of the recording sheet is tensioned towards both end portions thereof. Thereby, it is possible to reduce wrinkles being generated on the recording sheet.
According to the above-described fixing device, however, only the heat frame has the concave shape, that is, the backup roller has the straight shape. Therefore, in order to make the above-described difference in pressures applied between the ceramic heater and the backup roller appropriate, an amount of depression of the heater frame needs to be made large. However, when the amount of depression of the heater frame is large, it is difficult to keep the cylindrical member straight during operation.